A representative material gas concentration control system for this kind of material evaporation system is shown in the patent documents 1 and 2 identified below, which comprises a mass flow controller arranged in an inlet line that introduces a carrier gas, a constant temperature bath to keep the tank where the material liquid is stored at a constant temperature, and a pressure sensor that is arranged in an outlet line to output the mixed gas comprising the material gas and the carrier gas and that measures a pressure, namely a total pressure, of the mixed gas.
This material gas concentration control system evaporates the material liquid constantly at a saturated vapor pressure with a temperature of the material liquid kept constant so as to make a partial pressure of the material gas constant, and controls the flow of the carrier gas so as to make the total pressure measured by the pressure sensor constant by the use of the mass flow controller. With this arrangement, since the concentration of a vapor component is expressed by the partial pressure of the vapor component divided by the total pressure of the system, the concentration of the vapor, i.e., the concentration of the material gas, is also considered to be constant because the partial pressure and the total pressure are constant.